The present invention relates to a thermosetting adhesive composition or, more particularly, to a thermosetting adhesive composition based on an epoxy resin and a nitrile rubber and suitable for use in the preparation of substrate plates for flexible printed circuit boards and related products thereof such as coverlay films, bonding sheets having a layered structure by utilizing excellent properties relative to the adhesive bonding strength, heat resistance and others even after a prolonged heating treatment at an elevated temperature.
Along with the remarkable progress in the technology of electronics, it is a trend in recent years that electronic instruments used in a variety of application fields such as communication and civil life are required more and more to be compact in size and light in weight with a high degree of integration of various functions. Many of modern electronic instruments are therefore constructed and assembled by using flexible printed circuit boards which can be mounted and enclosed in a relatively narrow space by utilizing the flexibility thereof so that the instruments can be constructed with efficient three-dimensional utilization of the space within the casing of the instrument leaving little spaces left unutilized. Accordingly, application fields of flexible printed circuit boards are expanding year by year as a composite part to exhibit the functions not only as a circuit wiring means but also as a cable and connector means.
The above mentioned flexible printed circuit board is manufactured from an integrally laminated sheet material consisting of a flexible base film of a plastic resin having high heat resistance and excellent electric and mechanical properties and a metal foil adhesively bonded to one surface of the base film with intervention of an adhesive layer by patterning the metal foil by a conventional method into the pattern of a wiring circuit to serve as a part of the respective electronic instrument. Thus, such a flexible circuit board is required to exhibit good adhesion between the base film and the metal foil in addition to the requirements for good flexibility, high heat resistance and excellent electric properties as well as workability into a printed circuit board.
It is sometimes the case depending on the mode of application that the thus circuit-patterned layer of the metal foil is protected by attaching and bonding a coverlay film thereto. As a trend in recent years, many of the flexible printed circuit boards are provided with a pressure-sensitive adhesive coverlay film thereon attached by pressing thereto with an object to protect the circuit wiring layer and to improve the flexibility characteristic. The coverlay film used in this way is made from an insulating plastic resin film coated on one surface with an adhesive in a semi-cured state and the adhesive surface before use is protected by attaching a releasable protective sheet. Coverlay films are required to have excellent storage stability of the adhesive layer in addition to the adhesive characteristic, heat resistance, electric properties and workability.
Further, a so-called bonding sheet is sometimes used in the preparation of, for example, a multilayered flexible printed circuit board by adhesively bonding two flexible printed circuit boards as described above. A bonding sheet is prepared by uniformly coating the releasing surface of a releasable sheet with an adhesive in a semi-cured state and then attaching another releasable sheet to the adhesive surface thus to sandwich an adhesive layer with two releasable sheets. The requirements for the performance of bonding sheets are similar to those for the coverlay films.
As is readily understood from the above given description, the requirements for the performance of the materials in the manufacture of flexible printed circuit boards, including the flexible base sheets for circuit boards, coverlay films and bonding sheets, cannot be satisfied without use of an appropriate adhesive. In this regard, various adhesive compositions are proposed in the prior art, for example, in Japanese Patent Publication 5-62156, Japanese Patent Kokai 63-297483, 4-206112 and 64-26690 and elsewhere as suitable for use in the products of flexible printed circuit boards including those based on a nylon resin and epoxy resin, acrylic resin and phenolic resin, polyester resin and epoxy resin, nitrile rubber and epoxy resin and so on. Those based on a nitrile rubber and an epoxy resin are known to be an adhesive of high adhesion capable of exhibiting a relatively good balance of various properties though not without problems because the nitrile rubber per se contains a large number of ethylenically unsaturated bonds in the molecular structure which are subject to degradation when the adhesive composition is prolongedly heated at an elevated temperature resulting in a great decrease in the peeling resistance.